PROJECT 4: In nonhuman primate models of SIV infection, several studies have shown the importance of early CTL responses directed against certain epitopic regions which may lead to pressurized escape mutation, and the failure of a vaccine study due to escape mutation highlights the importance of understanding the earliest adaptive T cell pressures on virus sequence. A virus transmitted has already been passaged and shaped by the immune responses of the original host, and selection, transmission, and reversion of CTL escape mutations can occur. In specific aim #1 we will longitudinally study, at an individual epitope-specific level, the fate of epitope-specific responses in relation to viral sequence in partner recipients also studied in Project 1. The transmitting virus and HLA type of the source party will be known, as will the HLA type of the recipient. We predict that in addition to the cost in replication capacity of a CTL escape ' mutation that will be examined in Project 1, the reversion of a CTL epitope will depend upon whether the escape mutation is recognized as a target epitope in the new host. In a detailed study of 30 HLA typed partner pairs, we will longitudinally follow recognition of HIV-gag regions rich in CTL epitopes (p17andp24) using autologous peptides. In 10 subjects we will examine responses to whole genomes. For each pair, we will (a) determine the time to escape from wild type virus for epitopes not restricted by alleles shared with the source, (b) determine the time to reversion to wild type (or not)from escape mutant transmitted virus, and (c) ascertain whether reemergence of the escape mutation occurs ("re-escape"). Once transmission has occurred, subjects may continue to be exposed to new viruses. Available case reports suggest that superinfection occurs primarily in recent seroconverters, and superinfection has not been observed in large cohorts of persons with established infections. Our preliminary evidence suggests that T cell responses are present to some epitopes present in both the original and superinfecting virus, while other epitopes are present only in the original virus but are altered in the superinfecting virus. The quality of the CTL response may be critical in determining whether superinfection occurs. In specific aim #2 we will determine CTL responses to autologous epitopes in both the original and superinfecting viral variants to ascertain the characteristics of T cell responses which are effective or ineffective against the superinfecting virus. These results will be critical in advancing our ability to distinguish protective from ineffective CTL responses toHIV.